In contrast with internal combustion engines where the operating energy heating a working gas, indeed in part the operating gas itself, is derived from a chemical reaction or burning of a fuel within an operating gas space, hot gas engines, for example, Stirling cycle engines, drive operating thermal energy from a heat source external of the working gas space. Though more recently heat systems have been proposed utilizing heat derived from sources other than combustion in classical sense, even today usually the source is an external combustion carried out in immediate association with the engine, the thermal energy of which is supplied to working gas through a gas chamber wall either directly or by a heat transfer medium, whence the yet common designation "external combustion engines."
Whereas operation of an internal combustion engine is readily achieved with practical precision and good response time-wise, even for vehicular propulsion, as is evident in the typical fuel throttle controls used in various automobiles over many decades, for hot gas engines, engine power or torque output control through control of fuel supply, or other means of regulating combustion and heat developed, or by control of fluid whereby heat is transferred from a primary or secondary heat source to a point of transfer to the working gas, has generally been unacceptable for engines used for vehicle propulsion, especially in automobiles, because of inherent delay in the system and as well in some instances because of complexity of control device means required.
Hence, especially for hot gas engines used in automobiles, various systems of control have been proposed to obtain quicker response, for example, by changing the quantity of, or the pressure of, the working gas effectively present in the engine gas working spaces. But again the control systems adopted have entailed considerable complexity of structure, even where the speed and character of response has been otherwise acceptable. Likewise many arrangements have been proposed for torque and power control by change of the phase of displacers relative to their associated pistons.
However, even with the latter forms of control in an automobile, there arises the further disadvantage that, unless a clutch or other decoupling means is used between engine and drive wheels, then under conditions of a higher speed vehicle operation with comparatively low torque and power demand, as for example, running at constant higher speeds on a level road, nonetheless there are inherent windage type energy losses due to continued relative movement of displacers and working gas. A similar disadvantage is present for other engines continuously coupled directly to a load moving continuously but with variable speed torque, or power requirement.